doc: fix typo in examples for function bar_new (plugin API reference)
api: add argument "options" in function string_eval_expression, add option "-c" for command /eval (to evaluate a condition)
api: add support of colors with format "${color:xxx}" in function string_eval_expression and command /eval
doc: remove option ENABLE_DEMO for cmake in user's guide (demo plugin has been removed)
core: fix asciidoc formatting in ChangeLog
core: change color format for options from `${xxx}` to `${color:xxx}`

Options affected:
- weechat.look.buffer_time_format
- weechat.look.prefix_action
- weechat.look.prefix_error
- weechat.look.prefix_join
- weechat.look.prefix_network
- weechat.look.prefix_quit
core: replace "behaviour" by "behavior"
core: optimize creation of hashtable "pointers" in eval_expression if argument is NULL

The hashtable is created on first call to eval_expression with pointers == NULL.
On next calls, the hashtable is cleared, then used again.
The hashtable is freed on exit.
core: change colors of example in /help weechat.look.buffer_time_format
core: fix uninitialized variable "result" in callback of command /eval